1. Field of the Invention
The present invention relates to a system for detecting displacements of a sample surface from a reference point, and may be used, for instance, with autofocusing systems to focus a microscope to determine the thickness of films stretching over substrates and the width of wiring lines over circuit patterns or to position a laser head of an optical disk drive against an optical disk for reading and/or writing.
2. Description of the Prior Art
FIG. 1 is a schematic diagram of a conventional defocus detection system disclosed in Japanese Patent Laying Open Gazette 60-76035 (1985).
In this prior art system (hereinafter referred to as the "first prior art system", flux of light from a light source 21 is reflected by a half mirror 22, and travels through lenses 23 and 24 to form a spot on an optical disk 25. Flux reflecting from the spot travels through lenses 24 and 23 and the half mirror 22, forms an image, and finally reaches photo-sensors D.sub.1 and D.sub.2 which are obtained by dividing a single sensor into two parts.
An anterior louver 26 and a posterior louver 27, each with a knife edge and offset against the optical axis, are located symmetrically on both sides of point P.sub.10 at which an image is formed when the flux from the light source 21 is focused on the optical disk 25. When the optical disk 25 is on the focus, the light path is as illustrated by solid lines and the reflecting flux is not prevented by the louvers 26 and 27 from reaching the divided photo-sensors D.sub.1 and D.sub.2 and giving the photo-sensors D.sub.1 and D.sub.2 equal amount of light. Consequently, it is concluded that the optical disk 25 is on the focus when the photo-sensors D.sub.1 and D.sub.2 receive equal amounts of light, under which circumstances the imaging point P.sub.10 falls right in the middle of the louvers 26 and 27.
If the optical disk 25 is displaced off the lens 24 from the focus, an image is formed, for instance, at a point P.sub.11, as illusrated by broken lines in FIG. 1. The point P.sub.11 is closer to the anterior louver 26 than the point P.sub.10 and a part of the flux travelling above the optical axis towards the sensor D.sub.1 is blocked by the posterior louver 27. It is therefore concluded that the optical disk 25 is displaced off the lens 24 from the focus when the sensor D.sub.1 receives a decreased amount of light. Signals generated by the sensor D.sub.1 are fed back to an autofocusing mechanism to restore the optical disk 25 to the focus.
Contrarily, if the optical disk 25 is displaced towards lens 24 from the focus, an image is formed, for instance, at a point P.sub.12, as illusrated by two-dot chain lines in FIG. 1, the point P.sub.12 is closer to the posterior louver 27 than the point P.sub.10 and a part of the flux travelling above the optical axis towards the sensor D.sub.2 is blocked by the anterior louver 26. It is therefore concluded that the optical disk 25 is displaced towards the lens 24 from the focus when the sensor D.sub.2 receives decreased amount of light. Signals generated by the sensor D.sub.2 are fed back to the autofocusing mechanism to restore the optical disk 25 to the focus.
FIG. 2 is a schematic view of another conventional defocus detection system disclosed in Japanese Patent Laying open Gazette 58-60433 (1983).
In this prior art system (hereinafter referred to as the "second prior art system"), flux of light from a light source 31 travels through a beam splitter 32 and a lens 33 to form a spot on an optical disk 34. Flux reflecting from the spot travels through the lens 33 and reflects upon the beam splitter 32 towards an edge of a flat mirror 35. The flat mirror 35 is inclined at an angle of 45 degrees from the optical axis of the reflecting flux with the edge located at the convergent point of the light from the beam splitter 32. The reflecting light is diffracted at and radiated from the edge of the flat mirror 35.
Photo-sensors D.sub.5 and D.sub.6 receive a first set of equal amounts of light from the reflecting flux and photo-sensors D.sub.7 and D.sub.8 receive a second set of equal amounts of light when the optical disk 34 is on the focus. If the optical disk 34 is displaced off lens 33 from the focus, the sensors D.sub.5 and D.sub.8 receive decreased amounts of light. Signals thus generated are outputted to indicate that the optical disk 34 is displaced off lens 33 from the focus. Contrarily, if the optical disk 34 is displaced towards the lens 33 from the focus, the sensors D.sub.6 and D.sub.7 receive decreased amounts of light. Signals thus generated are outputted to indicate that the optical disk 34 is displaced towards lens 33 from the focus.
The first prior art system decides that a sample surface is on the focus when both sensors D.sub.1 and D.sub.2 receive equal amounts of light. However, if the sample surface has a high contrast reflectance which varies from place to place, as is the case with black and white stripe patterns, inequal amount of light may be incident on the sensors D.sub.1 and D.sub.2 even if the sample surface is on the focus. Therefore, there is a danger of concluding that the sample surface is not on the focus contrary to the fact.
The second prior art system decides that a sample surface is on the focus by comparing the amount of light received by the sensors D.sub.5 and D.sub.6 or D.sub.7 and D.sub.8 with each other. Again, there is a danger for a sample with a high contrast surface of misdetecting the focal state.
Both prior art systems also share the danger of mistake with transparent samples. Suppose a sample is a substrate made of a transparent material like glass. Then, undersired reflection from the back side of the sample (hereinafter referred to as back side reflection 11 may confuse sensors employed by the prior art systems and lead them to erroneous decisions. In other words, even if the front side of the sample is on the focus and reflection therefrom gives equal amounts of their light to each portion of respective sensors, the back side is off the focus and back side reflection is asymmetrically received by the sensors. Total amount of light received by each sensor concerned can not be equal. As long as comparison of the amount of light between two sensors provides the basis of decision, it is difficult to determine that the front surface of the sample is on the focus.
Other than these prior arts, which are referred to as "knife-edge methods", various other methods are known in the art. "Phase methods", which are disclosed in Japanese Patent Laying Open Gazettes 48-60645 (1973) and 60-37509 (1985), have a disadvantage in that it is difficult to defect objects having periodic patterns. In "contrast methods", which are disclosed in Japanese Patent Laying Open Gazette 59-232306 (1984), for example, astigmatism methods" which are disclosed in Japanese Patent Open Laying Gazette 50-99561 (1975), for example, "active methods" which are disclosed in Japanese Patent Open Gazette 58-217909 (1983) for example, and other methods, their are similar disadvantage.